What changes in cellular function account for the progression of breast cancer? One promising research area that addresses this fundamental question is the study of the interactions of insulin-like growth factor-II (IGF-II) and cathepsin D with the IGF-II/mannose-6-phosphate (M6P) receptor. Recent studies strongly suggest that complex interactions between these peptides and the IGF-II/M6P receptor are required for tumor growth and metastasis. Thus, elucidation of the regulatory mechanism(s) in which these cellular mitogens participate and their interactions with factors contributing to the progression of breast cancer will have direct applications for improvements in patient treatment and follow-up. IGF-II is a mitogen for breast cancer. Cathepsin D is an enzyme involved in metastasis. Both bind the IGF-II/M6P receptor at distinct binding sites. Nevertheless, a reciprocal inhibitory interaction is observed when IGF-II or cathepsin D bind the IGF-II/M6P receptor. Thus, intracellular IGF-II may cause increased cathepsin-D secretion instead of routing to the lysosomes. They hypothesize that intracellular as well as extracellular interactions of IGF-II and cathepsin D with the IGF-II/M6P receptor will promote tumor growth and metastasis. To test this hypothesis, they will determine whether (1) IGF-II expression modulates cathepsin D secretion in vitro, 2) oversecretion of proIGF-II/cathepsin D increases tumor growth and metastasis in vivo and 3) specific forms of IGF-II and cathepsin D are increased in breast tumor tissues and are associated with metastases in breast cancer patients. IGF-II and cathepsin D expression in transfected breast cancer cells and tumor tissues will be assessed by Northerns, Westerns and immunoprecipitation assays. Metastasis will be assisted in the SCID mouse model. A more complete understanding of the physiological consequences of the interactions of cathepsin D and IGF-II with the IGF-II/M6P receptor may have important therapeutic value as treatment targets for breast cancer and will provide new insights into the role in tumor development and progression. The research design integrates information obtained from characterization of the in vitro model system with the in vivo model and cathepsin D/IGF-II studies of normal and tumor tissues from breast cancer patients. More comprehensive studies may then be warranted to assess whether additional benefit is gained from including specific forms of cathepsin D and IGF-II among the prognostic indicators for tumor cell metastasis.